Fido’s No Doctor. Neither Is Whiskers.

A DOG or cat owner spends roughly $10,000 on the care and feeding of his pet over its lifetime. (Dogs cost more per year, but cats make up for it by living longer.) What does he get for this investment?

Surveys indicate that what most pet owners mainly want is companionship, unconditional love and a play pal. In recent years, however, we have also begun to regard pets as furry physicians and four-legged psychotherapists.

The idea that domestic animals are beneficial to human health and happiness has been fueled by books like “The Healing Power of Pets: Harnessing the Amazing Ability of Pets to Make and Keep People Happy and Healthy,” by the veterinarian Marty Becker, and by news reports claiming that having a dog helps you live longer or that swimming with dolphins can cure autism, bad backs, attention deficit disorder and even cancer. But is there any truth to these claims?

The task of distinguishing hype from reality on this question falls to anthrozoology, the new science of human-animal relationships. In 1980, Erika Friedmann, a scientist at the University of Pennsylvania, found the first evidence that animals might provide medical benefits: a survey of 92 heart attack victims revealed that those who had pets were nearly five times more likely to be alive a year later than those without them.

Since then, research has shown that stroking an animal lowers blood pressure, that AIDS patients living with pets are less depressed and that pet owners have lower cholesterol levels, sleep more soundly, exercise more and take fewer sick days than non-pet owners. Indeed, I have a stack of articles in my office supporting the hypothesis that pets are healthy for us.

Unfortunately, however, I also have another stack of articles, almost as high, showing that pets have either no long-term effects or have even adverse effects on physical and mental health.

A 2006 survey of Americans by the Pew Research Center, for instance, reported that living with a pet did not make people any happier. Similarly, a 2000 Australian study of mortality rates found no evidence that pet owners lived any longer than anyone else. And last year Dutch researchers concluded that companion animals had no effect on their owners’ physical or mental well-being. Worse, in 2006, epidemiologists in Finland reported that pet owners were more likely than non-pet owners to suffer from sciatica, kidney disease, arthritis, migraines, panic attacks, high blood pressure and depression.

An error has occurred. Please try again later.

You are already subscribed to this email.

This pattern of mixed results also holds true for the widely heralded notion that animals can cure various physical afflictions. For example, a study of people with chronic fatigue syndrome found that while pet owners believed that interacting with their pets relieved their symptoms, objective analysis revealed that they were just as tired, stressed, worried and unhappy as sufferers in a control group who had no pets. Similarly, a clinical trial of cancer patients undergoing radiation therapy found that interacting with therapy dogs did no more to enhance the participants’ morale than reading a book did.

As for the presumed curative powers of swimming with dolphins, researchers at Emory University who reviewed the dolphin therapy studies concluded that every one purporting to document positive health effects was methodologically flawed.

Don’t get me wrong. I don’t mean to disparage animal companionship; pets are central to my life, too. But the truth is that we know little about how pets could affect us biologically, or why a health benefit accrues to some people but not others. Answering these questions will require the same rigorous methods that scientists use to test the effectiveness of drugs and medical procedures.

Despite the importance of pets in our lives, researchers in the health and behavioral sciences have, until recently, largely neglected the study of human-animal relationships. But this is changing. In 2008, the National Institutes of Health (in conjunction with Mars, the corporate giant whose products include pet food) began a multimillion-dollar research initiative that will eventually help separate fact from wishful thinking on how pets influence human health and happiness.

No doubt, the talk in some medical circles of prescribing puppies and kittens for the chronically ill is well intentioned. But until the research is complete, pet lovers should probably keep taking their Lipitor and Prozac.

Hal Herzog, a professor of psychology at Western Carolina University, is the author of “Some We Love, Some We Hate, Some We Eat: Why It’s So Hard To Think Straight About Animals.”

A version of this op-ed appears in print on January 4, 2011, on Page A21 of the New York edition with the headline: Fido’s No Doctor. Neither Is Whiskers. Today's Paper|Subscribe